Thyristors can be used in situations where it is necessary to control high voltages and currents with a small bias voltage. Example applications include the switching of power into the electrical grid from new types of power sources, e.g., solar and wind generation devices or systems, and power quality control in the electrical grid using a FACTS (flexible AC transmission system).
A thyristor includes three p-n junctions. Thyristors are pnpn or npnp devices, and known used devices require more than one bias. The thyristor main bias is for anode-to-cathode power flow and another bias is for the thyristor gate. Additional biases may be required for switches that are auxiliary to the thyristor. FIGS. 1A-1C illustrate known electrically triggered thyristor devices, each of which requires a gate bias and a anode-cathode bias. Additional low voltage bias can be required for the switches. FIG. 1A is an integrated gate-commutated thyristor (IGCT), as commonly used to switch electrical current in industrial equipment. The low voltage applied to the gate-control terminal is used to switch the thyristor on and off. FIG. 1B is an emitter turn-off thyristor (ETO) that is typically implemented in SiC and used in power switching. FIG. 1C is a MOS turn-off transistor, also used in power switching The switches used in the prior art thyristors are parallel MOSFETs. Many parallel MOSFETs are used as a switch because of the high powers switched through the thyristors. The parallel MOSFET arrangement provides fast turn-off. In power conversion, the speed of device switching is an important concern to designers.
The need for multiple low voltage control signals causes design and reliability problems. The low voltage for control is typically less than 10V, whereas the high voltages being switched are on the order of kilovolts, e.g., 20 kV. Isolation of the low voltage control and high voltage signals can be problematic.
One method to isolate the circuits is to use an optical interface to receive the control signals. The optical control signals are still converted, such as by a photodetector to the low control voltage level for bias. The optical delivery of the control signal can effectively isolate control circuitry from the high power signals, but photodetectors introduce delays and signal-to-noise issues. The use of a photodetector typically also makes the control a binary operation. The control signal is supplied to the gate or base of the power semiconductor device via the photodetector to turn the power device on and off.
An example of a photodetector controlled device is described in Zhang et al., U.S. Patent Application No. US 2010/0283529. The thyristor disclosed in Zhang et al. can be controlled with an LED and a photodetector as in FIG. 4. However, Zhang's et al. thyristors requires first and second low control voltage biases, V1 and V2 as in FIG. 3. In the FIG. 4 device light applied to an optically triggered driver device, but multiple bias signals are still required.